A. Wireless Location
The process of determining the physical location of devices that emit radio frequency (RF) energy is known as geolocation. Many techniques exist for geolocation of RF emitters. A common geolocation technique is known as time-difference-of-arrival (TDOA). Classically, geolocation via TDOA is accomplished by simultaneously acquiring the RF emitter's signal at a multiplicity of sensors that are at different, and known, locations. The TDOA between any pair of the multiplicity of sensors is the difference in the time it takes the RF energy to propagate from its point of origin to each of the two sensors. The measurement of the TDOA between two sensors in two dimensions of known location yields a hyperbola with the two sensors coincident with the foci of the hyperbola. The hyperbola yields a multiplicity of locations that the RF energy could have emanated from. Deriving multiple hyperbolas from other pairs of sensors will produce a unique location from which the RF energy emanated. Geolocation of a RF emitter with TDOA in two dimensions requires that the signal be received with a sensor at a minimum of three distinct geographic locations. Each pair of sensors yields a hyperbola as the potential source of RF energy. Geolocation of a RF emitter with TDOA in three dimensions requires that the signal be received with a sensor at a minimum of four distinct geographic locations. Each pair of sensors yields a hyperboloid as a surface as the potential source of RF energy.
Early work relating to Wireless Location Systems is described in U.S. Pat. No. 5,327,144, Jul. 5, 1994, “Cellular Telephone Location System,” which discloses a system for locating cellular telephones using time difference of arrival (TDOA) techniques. The '144 patent describes what may be referred to as an uplink-time-difference-of-arrival (U-TDOA) cellular telephone location system. The described system may be configured to monitor control channel transmissions from one or more cellular telephones and to use central or station-based processing to compute the geographic location(s) of the phone(s). For example, in station-based processing, which may be employed for reverse control channel signal detection, cross-correlations are performed at the cell sites (or signal collection systems). For each “strong” signal, which may be considered a reference signal, received on a particular control channel at a particular first cell site, that strong signal is first applied to a signal decoder, such as that used by the cellular system itself. This decoder demodulates the cellular signal to produce the original digital bit stream which had been modulated to produce the cellular signal. This digital bit stream is then modulated by the cell site system to reconstruct the original signal waveform as it was first transmitted by the cellular telephone. This reconstructed signal waveform is cross-correlated against the received signal at the first cell site. The cross-correlation produces a peak from which an exact time of arrival can be calculated from a predetermined point on the peak. The first cell site system then sends the demodulated digital bit stream and the time of arrival to the central site over the communications line. The central site then distributes the demodulated digital bit stream and the exact time of arrival to other cell sites likely to have also received the cellular transmission. At each of these other second, third, fourth, etc., cell sites, the digital bit stream is modulated by the cell site system to reconstruct the original signal waveform as it was first transmitted by the cellular telephone. This reconstructed signal waveform is cross-correlated against the signal received at each cell site during the same time interval. The cross-correlation may or may not produce a peak; if a peak is produced, an exact time of arrival (TOA) can be calculated from a predetermined point on the peak. This TOA is then sent to the central site, and a delay difference, or TDOA, for a particular pair of cell sites can be calculated. This method permits the cell site systems to extract TOA information from an extremely weak signal reception, where the weak signal may be above or below the noise level. This method is applied iteratively to sufficient pairs of cell sites for each strong signal received at each cell site for each sample period. The results of the delay pairs for each signal are then directed to the location calculation algorithm.
An example of a wireless location system (WLS) of the kind described above is depicted in FIG. 1. As shown, the system includes four major subsystems: the Signal Collection Systems (SCS's) 10, the TDOA Location Processors (TLP's) 12, the Application Processors (AP's) 14, and the Network Operations Console (NOC) 16. Each SCS is responsible for receiving the RF signals transmitted by the wireless transmitters on both control channels and voice channels. In general, an SCS (now sometimes called an LMU, or Location Measuring Unit) is preferably installed at a wireless carrier's cell site, and therefore operates in parallel to a base station. Each TLP 12 is responsible for managing a network of SCS's 10 and for providing a centralized pool of digital signal processing (DSP) resources that can be used in the location calculations. The SCS's 10 and the TLP's 12 operate together to determine the location of the wireless transmitters. Both the SCS's 10 and TLP's 12 contain a significant amount of DSP resources, and the software in these systems can operate dynamically to determine where to perform a particular processing function based upon tradeoffs in processing time, communications time, queuing time, and cost. Each TLP 12 exists centrally primarily to reduce the overall cost of implementing the WLS. In addition, the WLS may include a plurality of SCS regions each of which comprises multiple SCS's 10. For example, “SCS Region 1” includes SCS's 10A and 10B that are located at respective cell sites and share antennas with the base stations at those cell sites. Drop and insert units 11A and 11B are used to interface fractional T1/E1 lines to full T1/E1 lines, which in turn are coupled to a digital access and control system (DACS) 13A. The DACS 13A and another DACS 13B are used in the manner described more fully below for communications between the SCS's 10A, 10B, etc., and multiple TLP's 12A, 12B, etc. As shown, the TLP's are typically collocated and interconnected via an Ethernet network (backbone) and a second, redundant Ethernet network. Also coupled to the Ethernet networks are multiple AP's 14A and 14B, multiple NOC's 16A and 16B, and a terminal server 15. Routers 19A and 19B are used to couple one WLS to one or more other Wireless Location System(s).
Geolocation techniques have become increasingly important to locate wireless devices as required by the Wireless Communications and Public Safety Act of 1999. Wireless devices present unique challenges to providing location information to emergency dispatchers, because they may not be in a fixed location. Enhanced 911 or E911 was developed as a feature of the 9-1-1 emergency calling system that automatically associates a physical address with the calling party's telephone number. Therefore, wireless devices may be located even if they are not in a fixed location.
E911 location techniques may include non-network-based location options for E911 Phase II that typically use the Navistar Global Positioning System (GPS) augmented with data from a landside server that includes synchronization timing, orbital data (Ephemeris) and acquisition data (code phase and Doppler ranges) as originally described in U.S. Pat. No. 4,445,118 (Taylor, et al).
Additionally, other wireless location techniques that generally cannot deliver E9-1-1 Phase II accuracies may be deployed in the carrier network to locate wireless devices. For example, these wireless location techniques may include forward channel techniques, such as EOTD (enhanced observed time difference of arrival), AFLT (advanced forward link trilateration) and enhanced cell-ID (ECID) where a wireless device collects the forward channel timings and/or signal strengths for relaying to an Serving Mobile Location Center (SMLC) or other landside server for location calculation. Additionally, non-wireless communication network techniques, such as the HDTV-based Rosum TV-GPS system described in U.S. Pat. No. 6,717,547, Apr. 6, 2004, “Position location using broadcast television signals and mobile telephone signals” and U.S. Pat. No. 6,522,297, Feb. 18, 2003, “Position location using ghost canceling reference television signals,” and the LORAN (LOng RAnge Navigation) may be deployed to locate wireless devices.
Furthermore, cell-based location techniques may be used to locate a wireless device. Inherent in the wireless carrier network are cell-based location techniques that may have been used to develop the position of the wireless. These techniques, also known as FCC E9-1-1 Phase I techniques, can generate a location based on the serving cell, the serving sector (if the cell is sectorized) or cell/sector with ranging (based on timing advance, ½ round trip time, or path-loss estimates).
B. Voice-over-Internet Protocol
Today, the Voice-over-Internet Protocol (VoIP) market is becoming increasingly popular as a less expensive alternative to traditional telephone services. Voice-over-Internet Protocol presents challenges similar to wireless devices in providing location information to emergency dispatchers, because a Voice-over-Internet Protocol adapter and the Voice-over-Internet Protocol network are not directly interconnected with the 9-1-1 network. Currently if an emergency services number is dialed from a commercial VoIP service, depending on how the VoIP provider handles such calls, the call may be connected to a carrier designated answering point, or it may be connected to a non-emergency number at the public safety answering point associated with the billing or service address of the caller. Because a VoIP adapter can be plugged into any internet connection with sufficient bandwidth, the caller may actually be hundreds or even thousands of miles away from the service address, yet if the call goes to an answering point at all, it may be the one associated with the caller's billing or service address and not the actual originating location.
Thus, in 2005, the Federal Communications Commission (FCC) passed an order requiring that Voice-over-Internet Protocol providers begin to provide E911 services to their customers. In this specification we will disclose methods and apparatuses for locating communication devices connected to a Voice-over-Internet Protocol telephone adaptor and transmitting the location to emergency services such as a universal Emergency Response number or short code [e.g. 9-1-1 (North America), 1-1-2 (EU), 9-9-9 (UK), 0-0-0 (Australia)] or the GSM wireless communication designated 1-1-2 global emergency number. Multiple individual numbers may be used dependent on the country of operation with police, fire, ambulance, civil defense, and public utilities known as common examples of these alternate emergency services numbers. Use of any of these numbers may be pre-programmed into the example embodiment. Use of non-mandated numbers may require prior agreement with the wireless carrier. Illustrative embodiments may use the terms 9-1-1, and Public Safety Answering Point (PSAP) in place of the multiple dialed digit codes and answering services possible.